1. Field of the Invention
The present invention relates to a host-side receiving apparatus that performs contactless communication.
2. Description of Related Art
In recent years, portable information devices such as mobile phones, wireless tags, and contactless IC cards (hereinafter PICCs (Proximity Integrated Circuit Cards)) are gaining widespread use. By utilizing contactless communication capability of such information devices, electronic ticket gates for electronically examining tickets at stations or the like, electronic money, and so forth have been introduced.
As a terminal side of an information device performing contactless communication, a PICC is adopted, and a reader/writer (hereinafter PCD (Proximity Coupling Device)) is adopted as a host side, for example. In general, electric power used in a PICC is generated utilizing radiowave transmitted from a PCD. For example, an electromagnetic induction method may be adopted, where an AC voltage can be induced in the antenna coil of the PICC by bringing the PICC into an AC magnetic field which is generated by electric current supplied to an antenna coil of a PCD. The PICC is designed to convert the generated AC voltage to a DC voltage to drive its internal circuits.
The AC magnetic field generated in the PCD can be obtained by supplying a carrier wave of a predetermined frequency to the antenna coil of the PCD. By modulating the carrier wave, information can be transmitted from the PCD to the PICC. As a modulation method, amplitude shift keying modulation (ASK modulation) is adopted, for example.
Also, by changing load connected to a resonance circuit including the antenna coil of the PICC, for example, electric current that flows to antenna coils can be changed in the PCD side in which antenna coils are electromagnetically-coupled to each other. Load of the resonance circuit can be changed by switching a load switch. That is, load is changed with the load switch and amplitude of a carrier wave transmitted and received between the PCD and the PICC is changed so as to enable transmission of information from the PICC to the PCD.
By the way, the amplitude of a transmitted carrier wave changes also depending on a distance between the antenna coil of the PCD and the antenna coil of the PICC. The longer the distance, the carrier wave amplitude tends to become smaller. However, relation between the carrier wave amplitude and the distance differs between when the load switch is turned on and when the switch is off: for example, the carrier wave amplitude is affected less by the distance when the load switch is on than when it is off. Therefore, depending on the distance, magnitudes of carrier wave amplitudes of when the load switch is on and when the switch is off can change place with each other, and at a certain distance, the carrier wave amplitude of when the load switch is on and that of when the switch is off can agree with each other.
In such a condition, change in the carrier wave amplitude between when the load switch is on and when the switch is off becomes zero and information cannot be transmitted. To avoid occurrence of such a null, it is required to perform adjustment of the antenna, adjustment of tuning frequency, and the like. However, occurrence of a null cannot be avoided for all possible combinations of PCDs and PICCs just with such adjustments.
Japanese Patent Application Laid-Open Publication No. 2002-521758 discloses a technique that provides two receiving channels and selectively operates those receiving channels in accordance with selection criteria to enable contactless communication tolerant to interference. However, even the proposal of Japanese Patent Application Laid-Open Publication No. 2002-521758 cannot prevent a null that can be caused depending on the distance between antennas.